According to BurntLatke.com's fueltool you need to inject 57-58 PSI of propane into your meter.

Question... Are you still able to access your fan? If it's a small fan it might not move enough air to get a decent mix of fuel/air in such a large chamber..

If your open to trying another fan, I would recommend the personal fans from Wal-Mart. I just picked up 4 of these at a $1.67 USD a piece. Ive seen a few combustions which use these and they builders say they are great. I didn't believe it but these things ROCK! They will move your hair from 5ft away.

Crap I forgot about the 2.5" pipe.. It will fit but you will probably have to trim the blade's by about half. I'm guessing that it will still be way more efficient at moving the propane throughout the chamber then a PC fan.